A non-toxic drilling fluid (drilling mud), for use in rotary-type well-drilling operations, comprises a non-toxic base-oil consisting essentially of branched-chain paraffins which may or may not contain ester functionalities, the base-oil having: (i) between about 16 and about 40 atoms per molecule, (ii) a kinematic viscosity, at 40.degree. C., of between about 1 and about 16.5 centistokes; (iii) a flash point of at least about 90.degree. C.; and (iv) a pour point of less than about -40.degree. C. Preferably, the branched-chain paraffin used as the base-oil consists essentially of the dimer of 1-decene, which has a viscosity of about 5 centistokes at 40.degree. C. and which has 20 carbon atoms per molecule. The drilling fluid can be about 100 percent base-oil or can be an emulsion of the base-oil in water, in which case the amount of base-oil is at least about 30 volume percent, and is more preferably between about 65 and about 95 volume percent. The drilling fluid additionally includes various additives, including emulsifiers, viscosifiers, weighting agents, oil-wetting agents, densifiers, and/or fluid-loss preventing agents.

An environmentally friendly invert drilling fluid comprises a water-in-oil emulsion which includes (a) at least 50 volume percent of a low toxicity base oil, and (b) at least one additive selected from the group consisting of emulsifiers, viscosifiers, weighting agents, oil wetting agents and fluid loss preventing agents, at least about 25 volume percent of the base oil content of the drilling fluid being one or more linear alpha-olefins which have from about 14 to 30 carbon atoms.

A method of electrically logging subterranean wells using a conductive fluid includes a miscible combination of an oleaginous fluid, a polar organic solvent, an electrolytic salt, and a carbon dioxide buffer. The electrolytic salt or brine of the salt is present in a concentration sufficient to permit the electrical logging of the subterranean well. The polar organic solvent may be an oil soluble glycol or glycol ether such as ethylene glycol, diethylene glycol, propylene glycol, polypropylene glycol, and the like. The electrolytic salt or brine preferably includes a quaternary amine salt having the formula: ##STR1## wherein R is an alkyl having 8 to 18 carbon atoms, R' is 2 to 6 carbon atoms alkyl group, B is hydrogen, oxyalkyl having 1 to 4 carbon atoms or alkyl having 1 to 4 carbon atoms, A is hydrogen or alkyl with 1 to 4 carbon atoms, x+y=1 to 15, z is 0 to 3, and M is a counter anion; or combinations thereof. The carbon dioxide buffer is preferably a trialkylanol amine.

A method of electrically logging subterranean wells using a conductive double emulsion fluid includes a miscible combination of an oleaginous fluid, an emulsifier capable of forming a microemulsion, an emulsifier capable of forming an invert emulsion, and an electrolytic salt. A microemulsion is the continuous phase of an invert emulsion. The electrolytic salt or brine of the salt is present in a concentration sufficient to permit the electrical logging of the subterranean well. The fluid may additionally contain a polar organic solvent and a carbon dioxide buffer. The polar organic solvent may be an oil soluble glycol or glycol ether such as ethylene glycol, diethylene glycol, propylene glycol, polypropylene glycol, and the like. The electrolytic salt preferably is a quaternary amine salt having the formula: ##STR1## wherein R is an alkyl having 8 to 18 carbon atoms, R' is 2 to 6 carbon atoms alkyl group, B is hydrogen, oxyalkyl having 1 to 4 carbon atoms or alkyl having 1 to 4 carbon atoms, A is hydrogen or alkyl with 1 to 4 carbon atoms, x+y=1 to 15, z is 0 to 3, and M.sup.- is a counter anion; or combinations thereof. In such instances the quaternary amine salt serves both as the electrolyte and the microemulsifer. The carbon dioxide buffer is preferably a trialkylanol amine.